Corrosion in Closed Cooling system

Sep 16, 2022
12
Seattle
Boat Info
1999 Sea Ray Express Cruiser 215, Mercruiser EFI, Closed Cooling system, Alpha 1 Stern Drive
Engines
Mercruiser 5.0 EFI Alpha 1 Stern Drive
I bought a 1999 Sea Ray Express Cruise with the Mercruiser 5.0 EFI. It has a closed cooling system. I have been working on it for a while and changed the zincs, impeller, oil, and water separator filter. Topped off the radiator fluid. I hooked the water hose up to the stern drive seems to be sucking water up into the engine to cool the coolant. Yet the boat temp climbs over about 4 minutes to 200 degrees the alarm goes off and I shut the engine off. I put the boat in the lake and it did the same thing. So now, of course, I think the thermostat is bad and I pull the thermostat out, along with the housing and the inside of the housing for the thermostat is full of white corrosion build up. I have cleaned that out, but how do I clean the debris from the corrosion out of the rest of the cooling system?

The engine sounds amazing, so the issue hasn't spread.
 
There is cooling system cleaners you can get. But to remove the scale buildup, you might have to flush that block pretty good. Need to check the heat exchanger too. Get some end seals and take the ends off and clean that real good too.
 
Did you only change the impeller or the also the housing (as a "kit")? ANY scoring inside the housing can cause issues.

Muriatic acid can be used in the engine to clean things out - but do not use it in the heat exchanger. I'd be looking at the heat exchanger first, though.
 
It was the manifold, they are so rusted out, and blocking the flow
 
Yes, if it is closed cooling, the block should be fine. Your corrosion should be limited to the manifolds and heat exchanger. If the manifolds are that bad, I would definitely rod the heat exchanger. That is a reasonably easy DIY, and critical for effective cooling.

What puzzles me, though, is the corrosion in the thermostat housing. The thermostat should be in antifreeze and corrosion free??
 
@keokie antifreeze needs to be flushed and changed every so often too. It’s a glycol solution mixed with fresh water that gives the water a higher boiling point and a lower freezing point. However the chemicals are also electrolytic in nature and are in contact with dissimilar metals inside the engine. If you never change the fluid and flush that part of the system every so often it can lead to electrolysis within the fresh water side of the cooling system. The thermostat is a choke point so it will get gunked up and the cast iron of the manifolds will of course rust the fastest of other metals.
 
@keokie antifreeze needs to be flushed and changed every so often too. It’s a glycol solution mixed with fresh water that gives the water a higher boiling point and a lower freezing point. However the chemicals are also electrolytic in nature and are in contact with dissimilar metals inside the engine. If you never change the fluid and flush that part of the system every so often it can lead to electrolysis within the fresh water side of the cooling system. The thermostat is a choke point so it will get gunked up and the cast iron of the manifolds will of course rust the fastest of other metals.

You're right. And it is a '99, so maybe it has never, or rarely been changed. I haven't seen corrosion inside a cooling system since a POS Chevette of a friend in high school in the 80's. I think many antifreeze options are better today than the old green stuff from the last century.
 
It may also be this was a raw water cooled engine and someone added the closed cooling system to it.
 

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